|MadSci Network: Physics|
Hi Ben, Sounds like an interesting project. I checked with a friend who does surveying work. The electronic distance meter does have adjustments for temperature and pressure. For normal surveying work they are not used. The adjustments are used for extremes of temperature or pressure, such as work above 10,000 feet in elevation. Some surveying instruments make the adjustments automatically. Why is the adjustment needed? You are correct that laser light passes through clear air with very little interaction. The beam is only visible when dust or other particles are present to reflect the light back to your eye. However, the speed of light is only a constant in vacuum. The speed of light changes when it passes through a material, such as air. The ratio of the velocity of light in a vacuum to the velocity of light in a material is called the index of refraction. The density (number of atoms or molecules per cubic meter) of air changes with temperature and pressure. It is the change in the density of the air, which changes its index of refraction, and changes the speed of light in air. In the surveying equipment, the distance is measured by multiplying the speed of light (2.99792458 X 10 8 meters/second) by the time in seconds to get the distance traveled in meters. If the speed of light changes, the accuracy of the distance measurement also changes. The time interval that needs to be measured is very short. At 100 meters the time it takes the light to travel to the reflector and back is only 6 microseconds! Have you ever looked at a hot object, such as a metal roof, on a sunny day? The air above the object seams to shimmer. Some people call this heat waves. The hot metal is heating the air above it and causing localized changes in the density of the air. The differences in the index of refraction cause the light passing through the heated air to be diffracted or bent. The diffracted light causes the shimmering effect we see with our eyes. The heated air is acting like a lens. Because the heating of the air is somewhat random, it is very difficult to correct for variations caused by heat waves. Astronomers do the corrections by creating an artificial guide star with a laser beam. The laser beam is visible because it reacts with a layer of sodium atoms in the upper atmosphere. A computer looks at the image of the laser beam and bends the optical elements of the telescope to correct the shape of the laser beam. Because the light from the stars follows the same path as the light from the guide star, the image of the starlight passing through the atmosphere is corrected for distortions caused by heat waves in the air. Setting up an experiment to measure the speed of light could be interesting. You might try using the laser range finder on the EDM to measure differences in transit time through different materials, if your dad lets you use his equipment. Good luck with the project, Bob Novak
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